The present invention relates to optical systems. More particularly the present invention relates to an apparatus and method for calculating a piston between segmented optics, such that the piston is larger than the wavelengths of electromagnetic radiation collected by the segmented optics.
The next generation of space-based telescopes are aimed to follow trends of modern terrestrial telescopes in that the collector optics will likely be segmented. Modern terrestrial based telescopes, such as the twin Keck Telescopes on Hawaii's Mauna Kea, have segmented primary mirrors to reduce the weight of the primary mirrors. Segmented mirrors in general may be made relatively lighter than monolithic mirrors of similar size. Providing segmented mirrors that are relatively lightweight reduces problems associated with the mirrors becoming misshapen under their own weight.
Manufacturers of space-based telescopes have similar goals in providing telescopes with segmented optics that are relatively lightweight. The motivations of these manufacturers, however, lie less in reducing flexure in the optical systems and lie more in providing telescopes that are inexpensive to launch (i.e., light weight), that have relatively large primary apertures, and that are foldable to fit into conventional space launch vehicles. One type of space based segmented optics telescopes include multi-aperture telescopes or extended-aperture telescopes. Each sub-aperture telescope of a multi-aperture telescope is configured to collect a select portion of a wavefront. The select wavefront portions are typically transmitted from the sub-aperture telescopes to a beam combiner where the select wavefronts are combined to form an image of the object or scene that generated the wavefronts.
Of concern in both terrestrial based telescopes and spaced based telescopes that have segmented optics are piston displacements between the segments. In a multi-aperture telescope, piston displacements equate to at least one sub-aperture telescope being out of phase with other sub-aperture telescopes. Piston displacement between sub-aperture telescopes introduces aberrations into images formed by the multi-aperture telescope.
Several techniques have been proposed to reduce piston in multi-aperture telescopes including various phase retrieval and phase diversity techniques that provide phase front information for collected wavefronts. From the phase front information, information of the relative piston between sub-aperture telescopes may be derived. For example, collected phase front information may indicate a given sub-aperture telescope is a half-wavelength (half-wavelength of collected electromagnetic radiation) advanced or retarded with respect other sub-aperture telescopes. Traditional phase diversity techniques, however, often do not provide piston information greater than one wavelength of collected radiation. For example, a relative displacement of two and a half wavelengths between sub-aperture telescopes would be indicated as a half-wavelength displacement using traditional phase diversity techniques. Accordingly, reducing relative piston greater than a wavelength of collected radiation tends to be complicated.
Accordingly, new techniques and apparatus are needed that provide for simplified phasing of a segmented optical system, wherein a piston between the optical segments exceeds the wavelengths of collected radiation.